A fixing apparatus of a heat roller system to be mounted on one of a printer and a copier of an electrophotographic system includes a halogen heater, a fixing roller heated by the halogen heater, and a pressure roller brought into contact with the fixing roller to form a nip portion. In addition, a fixing apparatus of a film heating system includes a heater including a heat generating resistor on a substrate made of ceramics, a fixing film moving while being held in contact with the heater, and a pressure roller forming a nip portion with the heater through the fixing film. In each of the fixing apparatus of the heat roller system and the fixing apparatus of the film heating system, while a recording material bearing an unfixed toner image is pinched and conveyed at the nip portion, the toner image is heated and fixed onto the recording material.
Regarding the above-mentioned fixing apparatuses, it is known that, when printing is performed on a small sized recording material continuously at the same print interval as the interval in a case of a large sized recording material, temperature rises extremely in a non-sheet feeding area (hereinafter, referred to as temperature rise at a non-sheet feeding portion).
The temperature rise at the non-sheet feeding portion is more likely to occur as process speed of the printer gets faster. The reason is that an intensive increase in speed is accompanied by shortening in time for the recording material to pass the nip portion and, therefore, fixing temperature required for heating and fixing a toner image onto the recording material is made frequently higher. When the temperature rise at the non-sheet feeding portion occurs in this way, respective parts configuring the fixing apparatus may be damaged. In addition, when printing is performed on a large sized recording material in a state of the temperature rise at the non-sheet feeding portion, toner is melted more than necessary in an area corresponding to the non-sheet feeding area in the recording material. Therefore, high-temperature offset takes place.
For the purpose of preventing the above-mentioned problem, as a unit of reducing the temperature rise at the non-sheet feeding portion, a method of increasing a thermal conductivity in an axial direction of a pressure roller is known. An advantage is that positive improvement in heat transfer in a rubber layer formed in the pressure roller leads to promote heat transmission in the axial direction so as to moderate extreme temperature rise at the non-sheet feeding portion.
Japanese Patent Application Laid-Open No. 2005-273771 discloses a pressure roller provided with a rubber layer containing dispersed pitch-based carbon fiber. In such pressure roller, the thermal conductivity in the axial direction of the rubber layer is high, and hence the temperature rise at the non-sheet feeding portion is effectively moderated.
The pressure roller disclosed in Japanese Patent Application Laid-Open No. 2005-273771 is excellent in the thermal conductivity in a roller axis direction. However, simultaneously, the pressure roller is turned out to give rise to a problem that the thermal conductivity in a thickness direction of the rubber layer is increased and heat is easily transferred from the rubber layer to the metal core so that a surface temperature of the pressure roller is likely to decrease. In a case where the surface temperature of the pressure roller is extremely low, moisture generated when the recording material passes a nip portion is likely to form dew on the surface of pressure roller to unstabilize conveyance of the recording material.
Further, even in a method of dispersing an needle-shaped filler having high thermal conductivity in a rubber layer, the rubber layer is required to have a certain amount of thickness in order to ensure satisfactory heat transfer in the roller axis direction while preventing hardness of the rubber layer from being extremely high. As a method of manufacturing a pressure roller which has a thickness enough to contain the needle-shaped filler and includes a resin tube layer excellent in die-releasing property as a surface layer, there is known a method of placing a metal core at the center of a molding die having a cylindrical inner surface, placing the resin tube on the inner surface of the molding die, and injecting liquid-state rubber between the metal core and the resin tube.
However, a step of applying primer on an inner surface of the resin tube is necessary. As a result, the number of steps of manufacturing the pressure roller is increased.